1. Field of the Invention
The present invention relates to a method and apparatus to enable wide bandwidth coverage of an optical spectral region with a temporal sequence of several narrowband optical frequency chirped pulses that are each frequency shifted.
2. Description of the Related Art
Optical linear frequency modulation (LFM) signals, i.e., frequency sweeps or “chirped” signals (also called “chirps”) have many uses in optical devices and processors. For example, chirps can be used to generate optical signals, to interact with optical signals, and to probe the optical spectral contents of devices or materials.
In a recent approach described in Merkel I, a temporally extended chirp is used as a probe waveform to generate a readout signal that represents a temporal map of the structure of the spectral population grating in an inhomogeneously broadened (IBT) material, rather than its Fourier transform. This temporal map signal can be measured with inexpensive, high-dynamic-range, MHz bandwidth detectors and digitizers. Such chirps generally have a duration greater than the decoherence time and less than the population decay time of the inhomogeneously broadened absorption spectrum in IBT material. As described in Merkel I, a chirp sweeping over some wideband portion of the IBT frequency absorption profile of interest e.g., typically in excess of 1 GHz can produce a low-bandwidth readout signal that can be detected and digitized with the low-bandwidth high-dynamic-range devices currently available. This low-bandwidth readout signal represents a temporal map of the spectral features in the spatial-spectral grating. For example, in some cases the readout signal includes a temporal spike that represents a single frequency hole burned in the IBT material, and in other cases the readout signal includes a superposition of low-bandwidth beat frequencies, each beat related to a periodic component in the frequency spectrum of the grating.
However, current known techniques for producing spectrally pure, phase continuous radio frequency chirps that are linear in frequency and very stable are limited to pulses with bandwidths less than about 400 MHz. For example, direct digital synthesis (DDS) of radio frequency signals with a digital to analog converter (DAC) produces a given sample rate and with given bits in the resolution for each sample. For example, with currently available state of the art DDS techniques, using a high clock rate and high-dynamic-range digital to analog converters (e.g., AD9858, 10 bit DDS at 1000 Ms/s, available from Analog Devices of Norwood, Mass., 1 Ms=a Mega sample=106 samples; a bit denotes the number of base 2 levels in a quantized system, i.e., 10 bits=210 quantized levels), a linear, repeatable, stable and phase continuous RF chirped pulse can be created with a bandwidth covering less than about 500 MHz (e.g., 0 to 400 MHz). The RF chirp can be impressed on an optical signal using an optical modulator such as an electro-optical modulator (EOM) or an acousto-optic modulator (AOM).
EOMs modify the optical carrier with double sidebands one above and one below the carrier frequency. AOMs shift the frequency of the optical beams and deflect optical beams by an angle proportional to the frequency shift. Optical beams can be double passed through an AOM to double the bandwidth of the RF signal, and remove angular deflections in the modulated beam. AOMs can be made single sideband easily because the carrier and second sideband beam are in different directions than the modulated beam with the first sideband.
Optical rings with a frequency shifter are known to produce a wide range of optical frequencies from a given optical frequency. See for example, K. Shimnizu, T. Horiguchi, Y. Hoyamada, “Technique for translating light-wave frequency by using an optical ring circuit containing a frequency shifter,” Optics Letters, vol. 17, p1307, 1992; Takesue et al., “Stable Lightwave Frequency Synthesis Over 1-THz Span Using Fabry-Perot Cavity Containing Polarization-Rotation Elements and Actively Controlled Tunable Bandpass Filter,” IEEE Photonics Technology Letters, vol. 12, p79, 2000; the entire contents of each of which are hereby incorporated by reference as if fully set forth herein. Similar rings have been used to produce a sweep of discrete frequencies. See for example, M. Ishikawa, H. Yasaka, F. Kano, Y. Yoshikuni, “Optical frequency sweeper using an optical ring circuit with a tunable injection-locking filter,” IEEE Photonics Technology Letters, vol. 11, n12, p1668, 1999; and U.S. Pat. No. 5,786,930 by Takatsu et al., issued Jul. 28, 1998; the entire contents of each of which are hereby incorporated by reference as if fully set forth herein.
While some of these references claim to generate optical frequency sweeps, in every case the frequency shift (Δf) imparted to an input signal at each pass through the loop is much greater than the bandwidth of the input signal, which is a single optical frequency (an optical “tone”). Thus the optical “sweeps” generated in the references are not continuous in frequency, even for short frequency ranges, but are noticeably stepped, i.e., they produce a series of discrete frequencies. The frequency gaps they leave make them unsuitable for reading out most spectra programmed into an IBT material, or other applications of interest wherein quasi-continuous coverage of broad bandwidths with optical frequency chirps are desired. As used herein the terms quasi-continuous frequency sweeps (or quasi-continuous chirps) are used to indicate chirps that are continuous in frequency for a least short frequency ranges, to distinguish such sweeps or chirps from the discrete frequency sweeps of prior art approaches.
Based on the foregoing, there is a clear need for techniques to produce a wideband frequency chirp that does not suffer the disadvantages of prior art approaches. In particular, techniques are needed to generate a stable, linear frequency chirped pulse over frequency ranges of much greater than 1 GHz, with RF sources that have bandwidths that are presently less than 1 GHz. Additionally, or in the alternative, there is a particular need for a linear, stable wideband frequency chirp with a wide bandwidth and an appropriate chirp rate to create a temporal map of the entire frequency band of interest in an IBT material.
The past approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not to be considered prior art to the claims in this application merely due to the presence of these approaches in this background section.